Calcium looping (CaL), or the regenerative calcium cycle (RCC), is a second-generation carbon capture technology. It is the most developed form of carbonate looping, where a metal (M) is reversibly reacted between its carbonate form (MCO3) and its oxide form (MO) to separate carbon dioxide from other gases coming from either power generation or an industrial plant. For this reason, calcium looping is also known as carbonate looping. In the calcium looping process, the two species are calcium carbonate (CaCO3) and calcium oxide (CaO). The captured carbon dioxide can then be transported to a storage site, used in enhanced oil recovery or used as a chemical feedstock. Calcium oxide is often referred to as the sorbent.
Calcium looping is being developed as it is a more efficient, less toxic alternative to current post-combustion capture processes such as amine scrubbing. It also has interesting potential for integration with the cement industry, as well as waste incineration and waste-to-energy plants.
Basic concept
There are two main steps in CaL: Calcination: Solid calcium carbonate is fed into a calciner, where it is heated to 850-950 °C to cause it to thermally decompose into gaseous carbon dioxide and solid calcium oxide (CaO). The almost-pure stream of CO2 is then removed and purified so that it is suitable for storage or use. This is the 'forward' reaction in the equation above. Carbonation: The solid CaO is removed from the calciner and fed into the carbonator. It is cooled to approximately 650 °C and is brought into contact with a flue gas containing a low to medium concentration of CO2. The CaO and CO2 react to form CaCO3, thus reducing the CO2 concentration in the flue gas to a level suitable for emission to the atmosphere. This is the 'backward' reaction in the equation above.
Note that carbonation is calcination in reverse.
Whilst the process can be theoretically performed an infinite number of times, the calcium oxide sorbent degrades as it is cycled. For this reason, it is necessary to remove (purge) some of the sorbent from the system and replace it with fresh sorbent (often in the carbonate form). The size of the purge stream compared with the amount of sorbent going round the cycle affects the process considerably.
Background In the Ca-looping process, a CaO-based sorbent, typically derived from limestone, reacts via the reversible reaction described in Equation () and is repeatedly cycled between two vessels.
The forward, endothermic step is called calcination while the backward, exothermic step is carbonation.
A typical Ca-looping process for post-combustion CO2 capture is shown in Figure 1, followed by a more detailed description.
Flue gas containing CO2 is fed to the first vessel (the carbonator), where carbonation occurs. The CaCO3 formed is passed to another vessel (the calciner). Calcination occurs at this stage, and the regenerated CaO is quickly passed back to the carbonator, leaving a pure CO2 stream behind. As this cycle continues, CaO sorbent is constantly replaced by fresh (reactive) sorbent.Blamey, J.; Anthony, E.J.; Wang, J.; Fennell, P.S. 2010, The Calcium Looping Cycle for large-scale capture. Progress in Energy and Combustion Science vol. 36 (2) p. 260-279 The highly concentrated CO2 from the calciner is suitable for sequestration, and the spent CaO has potential uses elsewhere, most notably in the cement industry. The heat necessary for calcination can be provided by oxy-combustion of coal below.
Oxy-combustion of coal: Pure oxygen rather than air is used for combustion, eliminating the large amount of nitrogen in the flue-gas stream. After particulate matter is removed, flue gas consists only of water vapor and CO2, plus smaller amounts of other pollutants. After compression of the flue gas to remove water vapor and additional removal of air pollutants, a nearly pure CO2 stream suitable for storage is produced.
The carbonator's operating temperature of 650-700 °C is chosen as a compromise between higher equilibrium (maximum) capture at lower temperatures due to the exothermic nature of the carbonation step, and a decreased reaction rate. Similarly, the temperature of >850 °C in the calcinator strikes a balance between increased rate of calcination at higher temperatures and reduced rate of degradation of CaO sorbent at lower temperatures.
Process description
CaL is usually designed using a dual fluidised bed system to ensure sufficient contact between the gas streams and the sorbent. The calciner and carbonator are fluidised beds with associated process equipment for separating the gases and solids attached (such as cyclones). Calcination is an endothermic process and as such requires the application of heat to the calciner. The opposite reaction, carbonation, is exothermic and heat must be removed. Since the exothermic reaction happens at about 650 °C and the endothermic reaction at 850-950 °C, the heat from the carbonator cannot be directly used to heat the calciner.
The fluidisation of the solid bed in the carbonator is achieved by passing the flue gas through the bed. In the calciner, some of the recovered CO2 is recycled through the system. Some oxygen may also be passed through the reactor if fuel is being burned in the calciner to provide energy.
Provision of energy to the calciner
Heat can be provided for the endothermic calcination step either directly or indirectly.
Direct provision of heat involves the combustion of fuel in the calciner itself (fluidised bed combustion). This is generally assumed to be done under oxy-fuel conditions; i.e. oxygen rather than air is used to burn the fuel to prevent dilution of the CO2 with nitrogen. The provision of oxygen for the combustion uses much electricity and is associated with high investment costs. Other air separation processes are being developed.
The penalties of calcium looping may be reduced by providing the heat for the calcination indirectly. This can be done in one of the following ways: Combustion of fuel in an external chamber and conduction of energy in to the vessel Combustion of fuel in an external chamber and use of a heat transfer medium.
Indirect methods are generally less efficient but do not require the provision of oxygen for combustion within the calciner to prevent dilution. The flue gas from the combustion of fuel in the indirect method could be mixed with the flue gas from the process that the CaL plant is attached to and passed through the carbonator to capture the CO2.
One efficient way of transferring heat into the calciner is by means of heat pipes. The indirectly heated calcium looping (IHCaL) using heat pipes has high potential to decarbonize the lime and cement industry. The deployment of this technology with refuse-derived fuels would allow to achieve net negative CO2 emissions.
Recovery of energy from the carbonator Although the heat from the carbonator is not at a high enough temperature to be used in the calciner, the high temperatures involved (>600 °C) mean that a relatively efficient Rankine cycle for generating electricity can be operated.
Note that the waste heat from the market-leading amine scrubbing CO2 capture process is emitted at a maximum of 150 °C. The low temperature of this heat means that it contains much less exergy and can generate much less electricity through a Rankine or organic Rankine cycle.
This electricity generation is one of the main benefits of CaL over lower-temperature post-combustion capture processes as the electricity is an extra revenue stream (or reduces costs).
Sorbent degradation It has been shown that the activity of the sorbent reduces quite markedly in laboratory, bench-scale and pilot plant tests. This degradation has been attributed to three main mechanisms, as shown below.
Attrition Calcium oxide is friable, that is, quite brittle. In fluidised beds, the calcium oxide particles can break apart upon collision with the other particles in the fluidised bed or the vessel containing it. The problem seems to be greater in pilot plant tests than at a bench scale.
Sulfation Sulfation is a relatively slow reaction (several hours) compared with carbonation (<10 minutes); thus it is more likely that SO2 will come into contact with CaCO3 than CaO. However, both reactions are possible, and are shown below.
Indirect sulfation: CaO + SO2 + 1/2 O2 → CaSO4 Direct sulfation: CaCO3 + + 1/2 O2 → CaSO4 + CO2
Because calcium sulfate has a greater molar volume than either CaO or CaCO3 a sulfated layer will form on the outside of the particle, which can prevent the uptake of CO2 by the CaO further inside the particle. Furthermore, the temperature at which calcium sulfate dissociates to CaO and SO2 is relatively high, precluding sulfation's reversibility at the conditions present in CaL.
Technical implications Calcium looping technology offers several technical advantages over amine scrubbing for carbon capture. Firstly, both carbonator and calciner can use fluidized bed technology, due to the good gas-solid contacting and uniform bed temperature. Fluidized bed technology has already been demonstrated at large scale: large (460MWe) atmospheric and pressurized systems exist, and there is not a need for intensive scaling up as there is for the solvent scrubbing towers used in amine scrubbing.
Also, the calcium looping process is energy efficient. The heat required for the endothermic calcination of CaCO3 and the heat required to raise the temperature of fresh limestone from ambient temperature, can be provided by in-situ oxy-fired combustion of fuel in the calciner. Although additional energy is required to separate O2 from N2, the majority of the energy input can be recovered because the carbonator reaction is exothermic and CO2 from the calciner can be used to power a steam cycle. A solid purge heat exchanger can also be utilized to recover energy from the deactivated CaO and coal ashes from the calciner.Romeo LM, Abanades JC, Escosa JM, Pano J, Giminez A, Sanchez-Biezma A, et al. Oxyfuel carbonation/calcination cycle for low cost CO2 capture in existing power plants. Energy Conversion and Management 2008;49(10):2809–14 As a result, a relatively small efficiency penalty is imposed on the power process, where the efficiency penalty refers to the power losses for CO2 compression, air separation and steam generation. It is estimated at 6-8 % points, compared to 9.5-12.5 % from post combustion amine capture.Blamey J., Anthony EJ, Wang J, Fennell PS, The calcium looping cycle for large-scale capture, Progress in Energy and Combustion Science 2010
The main shortcoming of Ca-looping technology is the decreased reactivity of CaO through multiple calcination-carbonation cycles. This can be attributed to sintering and the permanent closure of small pores during carbonation.
Closure of small pores The carbonation step is characterized by a fast initial reaction rate abruptly followed by a slow reaction rate (Figure 2). The carrying capacity of the sorbent is defined as the number of moles of CO2 reacted in the period of fast reaction rate with respect to that of the reaction stoichiometry for complete conversion of CaO to CaCO3. As seen in Figure 2, while mass after calcination remains constant, the mass change upon carbonation- the carrying capacity- reduces with a large number of cycles. In calcination, porous CaO (molar volume = ) is formed in place of CaCO3 (.). On the other hand, in carbonation, the CaCO3 formed on the surface of a CaO particle occupies a larger molar volume. As a result, once a layer of carbonate has formed on the surface (including on the large internal surface of porous CaO), it impedes further CO2 capture. This product layer grows over the pores and seals them off, forcing carbonation to follow a slower, diffusion dependent mechanism.
Sintering